The present invention relates to molecular variants of the angiotensinogen gene. The present invention further relates to the diagnosis of these variants for the determination of a predisposition to hypertension and the management of hypertension.
The publications and other materials used herein to illuminate the background of the invention or provide additional details respecting the practice, are incorporated by reference and for convenience are respectively grouped in the appended List of References.
Hypertension is a leading cause of human cardiovascular morbidity and mortality, with a prevalence rate of 25-30% of the adult Caucasian population of the United States (JNC Report, 1985). The primary determinants of essential hypertension, which represents 95% of the hypertensive population, have not been elucidated in spite of numerous investigations undertaken to clarify the various mechanisms involved in the regulation of blood pressure. Studies of large populations, of both twins and adoptive siblings, in providing concordant evidence for strong genetic components in the regulation of blood pressure (Ward, 1990), have suggested that molecular determinants contribute to the pathogenesis of hypertension. However, there is no information about the genes actually involved, about the importance of their respective effects on blood pressure, or about their interactions with each other and the environment.
Among a number of factors for regulating blood pressure, the renin-angiotensin system plays an important role in salt-water homeostasis and the maintenance of vascular tone; stimulation or inhibition of this system respectively raises or lowers blood pressure (Hall and Guyton, 1990), and may be involved in the etiology of hypertension. The renin-angiotensin system includes the enzymes renin and angiotensin converting enzyme and the protein angiotensinogen (AGT). Angiotensinogen is the specific substrate of renin, an aspartyl protease. The structure of the AGT gene has been characterized (Guillard et al., 1989; Fukamizu et al., 1990).
The human AGT gene contains five exons and four introns which span 13 Kb. The first exon (37 bp) codes for the 5' untranslated region of the mRNA. The second exon codes for the signal peptide and the first 252 amino acids of the mature protein. Exons 3 and 4 are shorter and code for 90 and 48 amino acids, respectively. Exon 5 contains a short coding sequence (62 amino acids) and the 3'-untranslated region.
Plasma angiotensinogen is primarily synthesized in the liver under the positive control of estrogens, glucocorticoids, thyroid hormones, and angiotensin II (Clauser et al., 1989) and is secreted through the constitutive pathway. Cleavage of the amino-terminal segment of angiotensinogen by resin releases a decapeptide prohormone, angiotensin-I, which is further processed to the active octapeptide angiotensin II by the dipeptidyl carboxypeptidase angiotensin-converting enzyme (ACE). Cleavage of angiotensinogen by renin is the rate-limiting step in the activation of the reninangiotensin system (Sealey and Laragh, 1990). Several observations point to a direct relationship between plasma angiotensinogen concentration and blood pressure; (1) a direct positive correlation (Walker et al., 1979); (2) high concentrations of plasma angiotensinogen in hypertensive subjects and in the offspring of hypertensive parents compared to normotensives (Fasola et al., 1968); (3) association of increased plasma angiotensinogen with higher blood pressure in offspring with contrasted parental predisposition to hypertension (Watt et al., 1992); (4) decreased or increased blood pressure following administration of angiotensinogen antibodies (Gardes et al., 1982) or injection of angiotensinogen (Menard et al., 1991); (5) expression of the angiotensinogen gene in tissues directly involved in blood pressure regulation (Campbell and Habener, 1986); and (6) elevation of blood pressure in transgenic animals overexpressing angiotensinogen (Ohkubo et al., 1990; Kimura et al., 1992).
Recent studies have indicated that renin and ACE are excellent candidates for association with hypertension. The human renin gene is an attractive candidate in the etiology of essential hypertension: (1) renin is the limiting enzyme in the biosynthetic cascade leading to the potent vasoactive hormone, angiotensin II; (2) an increase in renin production can generate a major increase in blood pressure, as illustrated by renin-secreting tumors and renal artery stenosis; (3) blockade of the renin-angiotensin system is highly effective in the treatment of essential hypertension as illustrated by angiotensin I-converting enzyme inhibitors; (4) genetic studies have shown that renin is associated with the development of hypertension in some rat strains (Rapp et al. 1989; Kurtz et al. 1990); (5) transgenic animals bearing either a foreign renin gene alone (Mullins et al. 1990) or in combination with the angiotensinogen gene (Ohkubo et al. 1990) develop precocious and severe hypertension.
The human ACE gene is also an attractive candidate in the etiology of essential hypertension. ACE inhibitors constitute an important and effective therapeutic approach in the control of human hypertension (Sassaho et al. 1987) and can prevent the appearance of hypertension in the spontaneously hypertensive rat (SHR) (Harrap et al, 1990). Recently, interest in ACE has been heightened by the demonstration of linkage between hypertension and a chromosomal region including the ACE locus found in the stroke-prone SHR (Hilbert et al, 1991; Jacob et al, 1991).
The etiological heterogeneity and multifactorial determination which characterize diseases as common as hypertension expose the limitations of the classical genetic arsenal. Definition of phenotype, model of inheritance, optimal familial structures, and candidate-gene versus general-linkage approaches impose critical strategic choices (Lander and Botstein, 1986; White and Lalouel, 1987; Lander and Botstein, 1989; Lalouel, 1990, 1990; Lathrop and Lalouel, 1991). Analysis by classical likelihood ratio methods in pedigrees is problematic due to the likely heterogeneity and the unknown mode of inheritance of hypertension. While such approaches have some power to detect linkage, their power to exclude linkage appears limited. Alternatively, linkage analysis in affected sib pairs is a robust method which can accommodate heterogeneity and incomplete penetrance, does not require any a priori formulation of the mode of inheritance of the trait and can be used to place upper limits on the potential magnitude of effects exerted on a trait by inheritance at a single locus. (Blackwelder and Elston, 1985; Suarez and Van Eerdewegh, 1984).
It was an object of the present inventiion to determine a genetic association with essential hypertension. It was a further object to utilize such an association to identify persons who may be predisposed to hypertension leading to better management of the disease.